Minority women, specifically African Americans (AAs), are substantially more likely than Caucasians to develop early onset breast cancer (EOBC) that is frequently aggressive and has a poor prognosis. In addition, epidemiologic studies have shown that AA women do not enjoy the same protection conferred by early full- pregnancy and multiparity as do Caucasian women. Some of these studies show that the risks of pre- menopausal breast cancer and more aggressive forms of the disease are actually increased by parity and early pregnancy in AA women. The molecular basis for this racial disparity is not known, but it is likely that the highly proliferative state achieved during development of the lactational differentiation, without appropriate homeostatic balance produced by subsequent waves of apoptosis, could contribute to this increased risk. We hypothesize that racially disproportionate p53 variants with reduced or altered function contribute to breast cancer health disparities and lack of pregnancy protection in AA women. In particular we propose that a non-synonymous SNP that results in either a proline (P) or arginine (R) at position 72 of the p53 protein may be an important contributor to this health disparity. The frequency of the P allele, which has been shown to be a less potent inhibitor of oncogenic transformation in vitro and to have reduced capacity to induce apoptosis in vivo, is much higher (66.6%) in AA than in Caucasian women (23.3%). We will test this hypothesis using the one existing animal model that harbors a humanized p53 gene, obtained by targeted homologous recombination of exon 4, encoding either R or P at codon 72. In specific aim 1, we will use this animal model to investigate whether the P allele, which is more frequent in AA women, provides a reduced level of pregnancy- induced mammary cancer protection than the R allele, which is more prevalent in Caucasian women. In specific aim 2, using the same animal model, we will investigate if the P allele cooperates with HER-2 in development of aggressive, early onset disease. Overexpression of Her-2 is one of the most common genetic alterations found in tumors from young AA breast cancer patients. In specific aim 3, we will investigate if the p53 alleles interact with the IGF-1 pathway. IGF-1 is an important mediator of the effects of energy balance and is another putative target of molecular alteration during mammary tumorigenesis. Finally, in Aim 4 we will investigate whether our mechanistic findings in animal models can be substantiated in human samples obtained from AA breast cancer patients. It is anticipated that these novel studies will shed new light on the molecular basis for outcome disparities in breast cancer, particularly the interactions of biological risk factors that may contribute to development of aggressive, early onset disease. Since a better understanding of the mechanisms involved in breast carcinogenesis in minority populations carries the promise of improved prevention and treatment strategies, this project directly and innovatively addresses the goal of reducing and/or eliminating health disparities.